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Milk packaging bag structure type and film performance requirements

    Since milk is a fresh drink, it has very strict requirements on hygiene, bacteria, temperature and other aspects. Therefore, there are special requirements for the printing of packaging bags, which makes the printing of milk packaging film different from other printing technology characteristics. The selection of milk packaging film must meet the requirements of packaging, printing, processing, storage and transportation, and hygiene. At present, the commonly used film material is mainly polyethylene (PE) co-extruded film, which is a polyethylene resin through melt extrusion and blow molding.


Types of film for milk packaging
    According to its layer structure, it can basically be divided into three types.

    1. Simple packaging film
It is generally a single-layer film, which is produced by various polyethylene materials with a certain proportion of white masterbatch and film blowing equipment. This kind of packaging film has a non-barrier structure and is hot-filled by pasteurization (85°C/30min), with a short shelf life (about 3 days).

     2. Three-layer black and white co-extrusion packaging film
A high-performance composite film made of LDPE, LLDPE, EVOH, MLLDPE and other resins, co-extruded and blown with black and white masterbatch. The black masterbatch added in the heat-sealed inner layer plays a role in blocking light. This packaging film uses ultra-high temperature instantaneous sterilization and hydrogen peroxide sterilization, and its shelf life can reach about 30 days at room temperature.

    3. Five-layer black and white co-extrusion packaging film
The intermediate barrier layer (composed of EVA, EVAL and other high barrier resins) is added when blowing the film. Therefore, this packaging film is a high barrier aseptic packaging film with a long shelf life and can be stored for about 90 days at room temperature. Three-layer and multi-layer black-and-white co-extrusion packaging film has excellent heat-seal performance, light- and oxygen-proof properties, and has the advantages of low price, convenient transportation, small storage space, and strong practicability.

Performance requirements of polyethylene film for dairy products
In order to meet the requirements of milk filling and printing, there are mainly the following requirements for polyethylene film.

1. Smoothness
The inner and outer surfaces of the film should have good smoothness to ensure that it can be filled smoothly on the high-speed automatic filling machine. Therefore, the dynamic and static friction coefficient of the film surface should be relatively low, and the smoothness of the film is generally required to be 0.2~0.4 The performance is mainly achieved by adding a slip agent. After the film is formed, the slip agent migrates from the film to the surface and accumulates into a uniform thin layer, which can significantly reduce the friction coefficient of the film and make the film have good slipperiness. effect.

2. Tensile strength
Since the plastic film is subjected to mechanical pulling force from the automatic filling machine during the filling process, the film must have sufficient tensile strength to prevent it from being broken under the pulling force of the automatic filling machine. The use of LDPE or HDPE particles with a lower melt index during the film blowing process is very beneficial for improving the tensile strength of the polyethylene film.

3. Surface wetting tension
In order for the printing ink to spread, wet and adhere smoothly on the surface of the polyethylene plastic film, the surface tension of the film should reach a certain standard, and the corona treatment must be used to achieve a higher wetting tension, otherwise it will affect the ink on the film. The adhesion and firmness of the surface affect the quality of the printed matter. It is generally required that the surface tension of the polyethylene film should be above 38dyne, and it is better if it can reach above 40dyne. Since polyethylene is a typical non-polar polymer material, the molecular structure does not contain polar groups, and has high crystallinity, low surface free energy, strong inertness, and stable chemical properties. Therefore, the film material has better printability Poor, unsatisfactory adhesion to ink.

4. Heat sealability
The most worried thing about automatic film packaging is the problem of leaking and imaginary sealing, leading to bag breakage. Therefore, the film must have good heat-sealing and bag-making properties, good sealing properties, and a wide heat-sealing range to facilitate packaging When the speed changes, the heat-sealing effect is not greatly affected, and MLLDPE is often used as the heat-sealing layer to fully ensure the stability of the heat-sealing conditions and heat-sealability. That is to say, it is necessary to ensure heat sealing and smooth cutting so that the molten resin will not stick to the knife.

Adding a certain proportion of LLDPE in the film blowing process can significantly improve the low-temperature heat-seal performance of the film and the heat-seal performance of inclusions, but the addition of LLDPE should not be too large, otherwise the viscosity of the polyethylene film will be too high. Prone to sticky knife failure. For the structure design of the film, the packaging film of the corresponding structure can be selected according to the packaging content and its shelf life.

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